The current trend in electronics is to reduce the size and increase the number of components, e.g., integrated circuit chips (IC chips) mounted per unit area of a substrate, e.g., a printed circuit board (PCB). Generally, these components generate heat that, if allowed to build up, can cause failure of a component if its temperature exceeds some threshold level. Typically, heat sinks are used to accelerate the dissipation of thermal energy from these heat-generating components.
Generally, the heat sink is attached to the substrate with mechanical devices such as, e.g., rivets, clips, clamps, and threaded fasteners such that the heat sink is in alignment with and urged toward the heat-generating component. Surface irregularities in both the component surface and the opposing heat sink surface prevent perfect contact between them. The resulting air gaps act as a heat insulator and reduce the efficiency of heat transfer. Typically, a thermal interface material (TIM) is interposed between the component surface and the opposing heat sink surface in an attempt to displace the air, fill the irregularities in the surfaces, and improve the efficiency of heat transfer.
Generally, a TIM comprises a binder and one or more thermally conductive fillers. The overall thermal efficiency of an assembly comprising the heat-generating component, the TIM, and the heat sink is affected by the bulk thermal conductivity and the thickness of the TIM, as well as the ability of the TIM to wet-out the surfaces, displace air, and fill in the surface irregularities. Often a static pressure is applied to the assembly to urge the heat sink toward the heat-generating component, thus decreasing the thickness of the TIM and, in some cases, improving wet-out. However, practical considerations, such as the mechanical strength of the heat-generating component, limit the maximum allowable applied pressure.
Typically, increasing the loading of thermally conductive fillers in the binder increases the bulk thermal conductivity of the TIM. However, increasing the loading of filler also increases the viscosity of the TIM, thus increasing the static force and/or the time required to achieve a given reduction in thickness of the TIM. The higher loading may also prevent the TIM from reducing in thickness to the desired value under the applied pressure. Thus, despite increasing the bulk thermal conductivity of a TIM, increasing the filler loading could actually increase the thermal resistance of an assembly.